This invention relates to a coupling joint for plastic pipe and more particularly to such a pipe joint for use in a plumbing circuit subjected to widely varying ambient temperature.
Spinwelding of plastic parts is well known wherein one part is spun against the surface of another at a surface differential speed such that the surfaces fuse and form a weld joint upon cooling. Examples may be seen in the insertion of a plastic plug in one end of a plastic conduit to form a holding tube. Another example is a butt welding process involving spinning of one edge of a cylindrical part against the edge of another similarly shaped part until the engaging circuits fuse from the friction induced heat and thereafter solidify to form a weld-like bond. In most plumbing systems, however, one length of pipe is coupled to another by placing a sleeve around the abutting ends of the pipes, and applying a bonding agent therebetween. As a consequence, a minimum of pipe misalignment is allowable, and once the joint has been set it cannot be separated without destruction of the joint. An entire section of pipe may have to be removed to remove such a joint, which necessitates replacement by a repair section of pipe, and a minimum of two joints in place of the one being removed. In the case of plastic pipe coupling joints which include a sleeve over abutting ends of plastic pipe, an external clamping pressure is necessary to hold the pipe sections together axially and to produce a pressure between the sleeve and the outside diameter of the pipe ends to obtain a joint seal. Over prolonged periods of time, when the pipe sections are subjected to considerable temperature excursions, the plastic in the pipe sections shows a marked tendency to creep and assume a smaller diameter as it yields to the clamping pressure, thereby allowing leakage between the joint sleeve and the outside surface of the pipe.
A plastic pipe coupling joint is needed which maintains its seal over prolonged periods of time and wide excursions of operating temperature, which provides a backup seal, and which may be disassembled and reassembled while allowing an appreciable degree of misalignment between the pipe axes.